The invention relates to a discharge lamp equipped with
a gastight lamp vessel transparent for visible radiation filled with at least one gas and mercury for generating ultraviolet radiation during operation of the discharge lamp,
a luminescent screen comprising luminescent substances for converting the ultraviolet radiation into visible radiation.
The invention also relates to a compact fluorescent lighting arrangement.
A discharge lamp as mentioned in the opening paragraph is known for instance from U.S. Pat. No. 4,176,294. These so called fluorescent lamps are used on a very large scale in a wide variety of lighting applications. In many of these applications it is desirable for a user of the discharge lamp to be able to control its light output. In practice control of the light output of the lamp can for instance be realized by controlling the amplitude of the applied voltage that during operation is present over the series arrangement of the ballasting element and the discharge lamp. Control of this applied voltage can be realized making use of a phase cut dimmer. Alternatively, in case the discharge lamp is operated by means of an electronic ballast, the light output can also be decreased by changing the operating frequency of the electronic ballast. In both cases a decrease in light output is accompanied by an increase of the electrode temperature and thus also the electron temperature of the mercury plasma in the discharge lamp rises. This in turn results in an increased amount of mercury emission between 350 nm and 450 nm. In other words, in case the light output of the discharge lamp is decreased, the emission of mercury in the blue region of the spectrum is increased, causing a colour point shift into the blue. In contrast to this, incandescent lamps behave like black body emitters. That means that if the average amplitude of the applied voltage, that is present between the ends of the glow spiral is decreased (for instance by making use of a phase cut dimmer), the light output is decreased and the colour point of the emitted light becomes more reddish while the colour temperature is lowered. Generally the colour point shift into the red of incandescent lamps when the light output of such a lamp is decreased is appreciated as more pleasant by a user of the lamp than is the colour point shift into the blue associated with the decrease in light output of a discharge lamp of the fluorescent type.
The invention aims to provide a discharge lamp that, when its light output is decreased, shows a colour point shift into the red.
A discharge lamp as mentioned in the opening paragraph is therefore according to the invention characterized in that the luminescent screen comprises an organic Eu3+ coordination compound for converting the mercury emissions between 350 nm and 450 nm into red light.
In case the light output of a discharge lamp according to the invention is decreased, the mercury emissions between 350 nm and 450 nm will increase. However, a substantial part of these mercury emissions will be converted into red light by the organic Eu3+ coordination compound. As a result, the colour point of a discharge lamp according to the invention shifts into the red in a way that is similar to the colour point shift of an incandescent lamp. The colour point shift into the red is appreciated as more pleasant by users of the discharge lamp than is a colour point shift into the blue.
Preferably, to prevent degradation, the organic Eu3+ coordination compound being diluted in an organic polymer that is transparent for radiation with a wavelength higher 350 nm. It has been found that the organic polymer can conveniently be incorporated into the luminescent screen in the form of a coating on an inner wall or an outer wall of the lamp vessel. Similarly the coating can be applied to the inner surface of a bulb that surrounds the discharge vessel.
Polymethylmethacrylate has been found to be a very suitable organic polymer to be used in a discharge lamp according to the invention.
Good results have been obtained for discharge lamps according to the invention, wherein the organic Eu3+ coordination compound comprises a compound of general formula Eu(diketonate)3(diimin) and wherein the diketonate anion is chosen from the group formed by 4,4,4-trifluor-1-(2-thenoyl)-1,3-butanedione, 4,4,4-trifluor-1-(2-naphthyl)-1,3-butanedione, dibenzoylmethane, benzophenone-4-acetylacetonate (and derivatives in which part of the hydrogens are substituted by methyl groups) and acetophenone-4-acetylacetonate (and derivatives in which part of the hydrogens are substituted by methyl groups) and the diimin is chosen from the group formed by phenanthroline, diphenylphenanthroline and 2,2xe2x80x2-bipyridine.
Good results have also been obtained for discharge lamps according to the invention, wherein the organic Eu3+ coordination compound comprises a compound of general formula Eu(Rxe2x80x94COO)3(triphenylphosphinoxide)2 and wherein Rxe2x80x94COO is a carboxylate anion chosen from the group formed by benzophenone-4-carboxylic acid and acetophenone-4-carboxylic acid and their derivatives in which part of the hydrogen is substituted by methyl groups.
A discharge lamp according to the invention is very suitable to be used in a compact fluorescent lighting arrangement comprising a discharge lamp and equipped with
a housing connected to the discharge lamp,
a lamp socket connected to the housing and comprising electrical contacts, and
a ballast circuit comprised in the housing and coupled to the discharge vessel and to the electrical contacts. Since the diameter of the lamp vessel in the compact fluorescent lighting arrangement is relatively narrow, a decrease in light output of the discharge lamp is accompanied by a relatively large colour point shift. The use of a discharge lamp according to the invention makes sure that the colour point is shifted into the red and not into the blue as a result of this decrease in lamp voltage.